The invention relates to a system and an automated method for the coulometric detection of dissolved gases. The method was developed for CO.sub.2 using the theoretically most accurate method of detection, i.e., coulometry, but other detection schemes can be used. It is believed the most relevant prior art is set forth in the publications, Coulometric Total Carbon Dioxide Analysis for Marine Studies: Automation and Calibration: Marine Chemistry, 21 (1987) 117-133; and Coulometric TCO.sub.2 Analysis for Marine Studies; An Introduction Marine Chemistry 16 (1985) 61-82, both of which references are incorporated by reference into this disclosure in their entireties.
Briefly, in the prior art system, there were three gas streams, a pneumatic gas stream, a carrier gas stream and a calibration gas stream. A sample bottle was placed in a thermostated bath. A pneumatic gas pressurized the sample bottle which caused the sample fluid to fill a glass pipette. Excess fluid from the filled pipette flowed into a vessel which contained a pair of sensing electrodes, which when covered with sea water completed an electrical circuit signalling the pipette was full. The pipette was then pressurized with the pneumatic gas. The sample drained from the pipette and flowed into a stripper where it was acidified and degassed to form an analyte, namely CO.sub.2. A carrier gas carried the analyte to a coulometer where the CO.sub.2 was measured. Pure CO.sub.2 (99.995%) gas was used for system calibration.
In the inventive system disclosed herein, the volume of the seawater output from a water jacketed pipette to the stripper is determined gravimetrically. The sample bottle and the pipette are maintained at the same temperature. The sample stripper accommodates an auxiliary carrier gas flow in addition to the main flow. The auxiliary flow enters the stripper at the top and the main gas flow enters the stripper through the bottom. This ensures the quantitative recovery of analyte gases stripped from solution.
Further, in this system, a fourth gas stream called the head-space gas is used to pressurize the sample bottle and fill the pipette. The head space gas feature controls the gas composition of the head-space which develops in the sample bottle as the sample flows from the bottle into the pipette. For example, if the TCO.sub.2 content of surface seawater is measured, compressed air (330 ppm CO.sub.2) could be used as the head-space gas because surface seawater would already by equilibrated with air (atmosphere) containing 330 ppm CO.sub.2. After the pipette is full, the sample bottle headspace is opened to the atmosphere to equalize the pressure inside and outside the bottle. At this point, the pressure and composition of the head-space gas inside the bottle are 1 atmosphere and 330 ppm CO.sub.2, respectively. For a surface seawater equilibrated with the atmosphere as in the example above, the concentration of CO.sub.2 is equal in both the water and air (330 ppm) at one atmosphere of pressure, and by using a head-space gas of compressed air this relationship is maintained during the analysis so that no CO.sub.2 gas is likely to be forced into or out of the sample during the analysis in response to a concentration or pressure gradient. Note that only a small fraction of the TCO.sub.2 in seawater is in the molecular phase as CO.sub.2 gas, rather 99% appears in the ionic phase as bicarbonate or carbonate ions, but the coulometric titration is so sensitive that the small exchanges of molecular CO.sub.2 into or out of the head-space could be measured, and cause under or over estimates of the true TCO.sub.2 concentration. For seawater collected at depths .gtoreq.300 meters, a head-space gas enriched in CO.sub.2 (1000 ppm) would be used, but it is not necessary to custom fit the head-space gas to each sample because the exchange rate is slow compared to the analysis time and so much is known about the saturation status of seawater with respect to CO.sub.2. However, in practice and principle an independent CO.sub.2 containing head-space gas is demonstrably superior to the CO.sub.2 -free pneumatic gas used in the prior art. The above would also apply to other gases dissolved in seawater such as methane, hydrogen sulfide, etc. After the analysis is completed, the sample bottle is re-pressurized and the pipette re-filled and the cycle repeated for the next replicate.
A constrictor is placed in the line between the stripper and coulometer to create backpressure. Opening a solenoid valve causes the sample to immediately drain from the stripper. There is no possibility of a malfunction as happens from time to time with check valves because of spring failure, clogging, oxidation, condensation, etc. A second function of the restrictor is to prevent the coulometer cell solution from backing up into the input lines when momentary pressure differentials arise during operation.